If you haven't been listening to pop radio in the past few months, you've missed the rise of two seemingly opposing trends. In a medium in which mediocre singing has never been a bar to entry, a lot of pop vocals suddenly sound great. Better than great: note- and pitch-perfect, as if there's been an unspoken tightening of standards at record labels or an evolutionary leap in the development of vocal cords. At the other extreme are a few hip-hop singers who also hit their notes but with a precision so exaggerated that on first listen, their songs sound comically artificial,...